Re: problem with mh-e and 2002-08-20 change to font-core.el

[Richard Stallman]

    > With the current code, changing from Occur mode to Fundamental mode
    > and back to Occur mode would lose the font-lock-face properties.
    > That is a bug.
    > 
    > With the change you propose, changing from Occur mode to Fundamental
    > mode and then to Info mode would leave you with font-lock-face
    > properties left over from Occur mode.  That would be a bug.

    I'm not sure how important this is, really.

It isn't worth a large amount of work.

    but that might be run too late.  So maybe we should add some
    support in font-core.el to make it easier for a mode to tell
    "erase font-lock-face property when font-lock-mode is changed".

The whole point of the font-lock-face property is that it can be set
up unconditionally, and is there regardless of whether Font-Lock mode
is enabled.  It would be wrong to remove these properties when turning
off Font-Lock mode.  Only changing the major mode is a reason to
remove them.

    First, I think that the change-major-mode-hook should
    run (font-lock-mode -1) and that if it doesn't do the right thing,
    then we should fix (font-lock-mode -1) rather than change
    the change-major-mode-hook.

For the reason given above, this is simply wrong.

Perhaps the modes that use font-lock-face should set up
change-major-mode-hook to clear it out.  That would solve the
inconsistency that you pointed out, that the font-lock-face properties
won't be cleared unless someone actually turned on font-lock-mode.
Everything will work right except cases like switching to Occur mode,
and those cases are not worth a lot of work to fix.